I have Mega16 connected directly to SIM300C GSM-modem. The problem is that it is not working stable. I.m trying to make a voice or data call and receive 1-2 calls of 10-20 attempts. I have RX & TX connected to modem and CTS-RTS connected to each other. Also Frame Error (on Mega`s USART) occurs when trying to make a call (ATDxxxxxxxxxxx).
I have to mention that the modem and Mega16 works ok conected to PC via max232 on the same protocol and speed settings (9600, 8-N-1).
I.m using 8Mhz external crystal, 7805 stabilizer and 12V taken from ATX power block.
I suppose that some external pull-ups are required on RX,TX lines but have not found an example. If anybody have such a problem - please help me to sort it out.